This invention relates to an improved process for the production of vinyl chloride homo- and copolymers by suspension polymerization of vinyl chloride or copolymers of vinyl chloride and copolymerizable monomers which prevents to a great extent the formation of polymer encrustations on the inner reactor wall and on associated equipment.
In conventional processes for the production of vinyl chloride polymers by suspension polymerization in an aqueous medium, polymer encrustations are formed on the inner reactor wall as well as on the associated equipment during the course of the polymerization. The polymer yield is thereby reduced and the quality of the product is impaired due to intermixing with detached wall encrustations. These coatings also impede the removal of the heat of polymerization and accordingly have a deleterious effect on the space-time yield of the reactor.
Additionally, frequent cleaning of the reactors is necessary. For cleaning purposes, automated high-pressure water jet appliances are employed. However, such devices only remove the less tightly adhering wall deposits. For this reason, the reactor must be entered after a few batches and must be cleaned be hand with the aid of a scraper. This requires extensive safety measures and, in case of very large reactors, also the installation of scaffolding.
The above-described cleaning steps entail great expenses and frequent and long idle periods, whereby the economics of the process are severely impaired. Moreover, damage to the equipment surfaces can hardly be avoided during the manual cleaning with a scraper.
Numerous measures have become known for preventing the formation of polymer encrustations during the suspension polymerization of vinyl chloride. The prior art includes process-technological methods, such as
A. REGULATING THE JACKET TEMPERATURE TO THE INTERNAL TEMPERATURE OF THE REACTOR (German Unexamined Laid-Open Application DOS No. 1,520,609 corresponding to British Pat. No. 1,053,572);
B. LOW TEMPERATURE COOLING OF THE REACTOR WALLS (DOS No. 2,033,780);
C. EXPOSING THE INTERFACE AREA TO A SPRAY (DOS No. 2,224,144).
These proposed solutions, however, are not completely satisfactory since they require substantial technical expenditure. Also, the step of subjecting the walls to low-temperature cooling can be utilized only in the case of very large reactors.
It is also known to prepare the reactor wall with polar organic compounds, organic dyes and/or inorganic pigments, as well as to supplement the batch by additions of such compounds (DOS No. 2,044,259). However, such linings must be renewed after a few runs. Moreover, The substances washed off the wall and/or added to the charge deleteriously affect the quality of the polymerized product.
It has also been known to reduce the formation of encrustations by adding special agents to the polymerization charge or by using special polymerization recipes. These measures, however, lead only to partial success and furthermore are limited to specific polymerization formulae.
Thus, DOS No. 2,208,796 and DOS No. 1,946,474 (corresponding to U.S. Pat. No. 3,562,238). describe the addition of compounds having a basic reaction to the polymerization charge. As demonstrated in the examples set forth below (Table 1), this provides only a minor improvement, however.
The addition of nitrites in connection with the use of unbranched alkyl peroxydicarbonates as a measure to prevent wall deposits is described in DOS No. 2,225,236 (corresponding to U.S. Pat. No. 3,817,959). However, such additives lead to an increase in the operating time and moreover are not effective in all formulae, as demonstrated by the examples (cf. Table 1).
Several German applications disclose the addition of oxidizing agents, but in all cases merely as a supplement to another measure. Thus, DOS No. 2,044,259 (Claim 4) describes the addition of a permanganate, a dichromate and/or cerium(IV) sulfate. DOS No. 2,224,144 discloses (Claim 6) the addition of permanganic acid or the salts thereof, chromic acid, dichromic acid or the salts thereof, nitric acid, dinitrogen monoxide, dinitrogen tetroxide, dinitrogen trioxide, copper nitrate, lead nitrate, silver nitrate, ammonium nitrate, cerium oxide, copper oxide or lead oxide, in concentrations of at most 0.0001% by weight (Claim 8). These additives, taken by themselves, i.e., without any other supplemental measures, do not yield the intended success, as shown in the comparative examples (Table 1). The steps known from the prior art for preventing wall deposits during the suspension polymerization of vinyl chloride thus are unsatisfactory. They are either of little effectiveness or are connected with excessive costs. Furthermore, they have only limited applicability. Besides, part of the measures results in undesired residues of dyes and/or heavy metal compounds remaining in the polymer.
Accordingly, it is an object of the invention to provide an improved method for inhibiting the formation of encrustations during such polymerizations. Other objects will be apparent to those skilled in the art.